Method of making sensing elements for resistance-temperature probes

ABSTRACT

Resistance temperature probes having novel sensing elements are disclosed. The resistance element is not wound on a rigid support but is supported in a body of particulate, dielectric refractory material. The disclosure includes methods of fabricating the probes including calibration of the resistance in situ. An important feature resides in the method of incorporating the particulate refractory material into the probe. Since it is important to provide the most direct path for exchange of heat between the environment and the sensing element, this latter is allowed to be free-standing and the space between the same and the sheath, is filled with the refractory material. In order not to deform or displace the sensing element, the particulate material is deposited in a manner calculated to provide such support, but not to subject the delicate resistance wire to injurious forces.

United States Patent Mochizuki [54] METHOD OF MAKING SENSING ELEMENTSFOR RESISTANCE- TEMPERATURE PROBES [72] Inventor: Mitsuaki Mochizuki,Kobe, Japan [73] Assignee: Okazaki Manufacturing Company, Kobe,

Japan [221 Filed: Sept. 2, 1969 21 Appl.No.: 854,415

[30] Foreign Application Priority Data Sept. 6, 1968 Japan ..43/064097[52] US. Cl ..29/614, 29/619, 338/238, 338/247 [51] lnt. Cl. ..H0lc7/00, HOlc 17/00 [58] FieldofSearch ..29/610,6l1,6l3,614,619; 338/28,30, 238, 229, 240, 241, 243, 247, 273

[56] References Cited UNITED STATES PATENTS 579,459 3/1897 Whittingham..338/241 1,432,064 10/1922 Hadaway, Jr. ..338/241 X 2,091,839 8/1937Tangeman ..29/619 X 2,491,688 12/1949 Pickels ..29/613 2,508,512 5/1950Grinde ..338/229 2,635,163 4/1953 Temple ..29/619 X [4 1 Feb. 1, 19722,643,317 6/1953 Tuttle ..29/613 X 3,267,733 8/1966 Chambers. ..338/238X 3,434,207 3/1969 Frachon ..29/619 X FOREIGN PATENTS OR APPLICATIONS1,000,463 8/1965 Great Britain ..338/240 Primary Examiner-John F.Campbell Assistant Examiner-Victor A. DiPalma Attorney-Leonard S. Knox[57] ABSTRACT Resistance temperature probes having novel sensingelements are disclosed. The resistance element is not wound on a rigidsupport but is supported in a body of particulate, dielectric refractorymaterial. The disclosure includes methods of fabricating the probesincluding calibration of the resistance in situ.

- An important feature resides in the method of incorporating 1 Claims,11 Drawing Figures PATENTED FEB] 1972 Fla? FIGS Era/l F1015 F166 I FIG.5 F1119 FIG, i0

nlllllilnnlua /N vm TOR Mztsualci Mac/21211161 5 g .5. fyx

O'fly METHOD OF MAKING SENSING ELEMENTS FOR RESISTANCE-TEMPERATUREPROBES BACKGROUND OF THE INVENTION Heretofore, in the manufacture ofresistance-temperature probes it has been the practice to wind theresistance wire around a mica or glass mandrel and, optionally, toinsert the same in a glass tube so that the wire, which is of finegauge, has adequate support. Sometimes the element is fused to themandrel and/or tube. However, these techniques have the follow- SUMMARYOF THE INVENTION The present invention overcomes the foregoingshortcomings by eliminating the rigid support, whereby the probe may beuseful at temperatures substantially higher than the maximum heretoforepossible.

The invention probe includes a rigid sheath to contain the resistancewire element and to protect the same. A particulate refractory surroundsthe resistance wire. This refractory is not compacted, as has been priorpractice, but is merely densely deposited around the element and withinthe confines of the sheath, in such a way as to avoid pressure forces onthe wire, i.e., deposition is controlled so as to avoid the formation ofa rocklike mass restricting expansion and contraction of the resistancewire. However, the wire is given sufficient support to precludeshort-circuiting or grounding of its turns or convolutions. Theinvention probe has primary utility in applications where high accuracybut low resistance to shock and vibration are factors.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a metallic sheath portionand compacted refractory, dielectric material together with the leadwires, but with the resistance element not yet joined thereto;

FIG. 2 shows the element joined to the lead wires;

FIG. 3 shows a section of sheath united to the sheath portion in a buttjoint;

FIG. 4 shows the element and surrounding refractory and the end of thesheath section closed;

FIG. 5 shows an alternative embodiment wherein the sheath section isunited with the sheath portion by means of a telescoping joint;

FIG. 6 shows an alternative form in which the resistance wire is housedin a capsule of glassy material together with relatively uncompactedrefractory,

FIG. 7 is a cross section on the line 7-7 of FIG. 6;

FIG. 8 is similar to FIG. 4 but includes the capsule of FIG. 6;

FIG. 9 shows an alternative mode of supporting the resistance wire in arigid tube;

FIG. 10 shows the subassembly of FIG. 9 formed into a U- shape forcompactness; and

FIG. 11 is a cross section taken on the line IlII of FIG. 10.

DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS Adverting first to FIGS. 1to 5, there is shown a sheath portion 10 of any desired length,depending upon the application, and of any suitable metal, e.g.,stainless steel. The lead wires 12-12 are housed within the sheathportion 10 and are securely positioned by compacted refractory,dielectric material, e.g., powdered M 0, whereby the wires are insulatedfrom each other and from the sheath portion.

The sensing element 14 is shown in the form of unannealed resistancewire of the gauge and length required to provide the desired ohmicresistance and is wound into a helix. Since the element is to besubsequently calibrated by trimming, excess turns may be allowed. Thehelix is bent on itself to form a loop and the free ends are welded orotherwise secured to the ends of the leads l2l2 (FIG. 2). At this point,the element 14 is essentially self-supporting. It will be understoodthat the element may be of geometrical form other than that shown.

A sheath section 21 of essentially the same diameter as the sheathportion 10 is butted to the end of the latter and welded (FIG. 3). Thevoids within the sheath section 21 are densely filled with powdered M 0or equivalent refractory, which operation may be facilitated byvibrating the assembly. The time and intensity of vibration will becontrolled in such a way that the element is adequately supported butnot restricted against expansion and contraction, as was the case withthe rocklike mass characteristic of prior probes. The degree ofdenseness of the refractory will take into account acceptable conductionand convection of heat between the element and the environment of theprobe. A closure 23 is welded to the upper end of the sheath section 10.

The embodiment of FIG. 5 is similar to that of FIG. 4 except that, inthis case, the sheath section 2111 is larger and telescoped over thesheath portion, and is then welded.

In either fonn of the invention shown in FIGS. 4 and 5 swaging of theassembly may be resorted to in order to obtain a more dense matrix ofinsulation. It is to be noted that, within the framework of thedisclosure, undue compaction is avoided. This means that when therefractory is initially deposited within the sheath, there will beenough looseness to accommodate for this later swaging. The usualannealing is performed. Following such optional step the closure 23 isremoved in order to calibrate the resistance element 14 to the propervalue. This is accomplished by removing a portion of the insulation toexpose the bight 26 of the loop, which may then be cut and trimmed untilthe selected ohmic resistance is obtained. The cut ends are thenrejoined, the refractory replaced, if desired, and a new closure 23welded on.

In FIG. 6 the sensing element 14a is encapsulated in a piece of tubingof quartz or other fusible glassy material which is closed at one end,the powdered refractory deposited in a dense but not rocklike matrix andthe other end of the capsule 31 closed by heat while leaving the ends ofthe loop protruding for juncture with the leads l2l2. Since the glassymaterial is an insulator it is immaterial if the element touches thesides of the capsule. The sensing element subassembly of FIG. 6 islocated in a sheath section and the several steps described inconnection with FIGS. I to 4 or FIGS. I, 2 and 5 follow. In the case ofFIG. 6 it will be clear that the refractory surrounding the capsule 31need only be dense enough to center and support the capsule. Further,since swaging of the sheath section 21 and sheath portion 10 mayfracture the quartz envelope of the capsule, this step is desirablyeliminated.

FIGS. 9, l0 and 11 illustrate an alternative in which a helix 4] ofresistance wire is threaded into a quartz tube 42. The voids are thenfilled with powdered insulation, e.g., M 0. In this case also the helixmay contact the interior of the tube. After the ohmic resistance of theelement is adjusted by trimming, the ends of the tube are fused to embedthe leads 4444. Then the center region of the tube is heated and thesame bent into the U-form of FIG. 10. The subassembly of FIG. 10 may nowbe incorporated in the probe as described in connection'with FIGS. 1 to4.

From the foregoing it will have become apparent that the only restrainton the sensing element is the surrounding body of powdered refractory,dielectric material, for example, M,,0. When the latter is selected tohave a very high degree of purity, it will be found that the insulationresistance is very high, even at high temperatures, the thermalconductivity is excellent and the element is not subject to degradationby contaminating gases.

device which includes a metallic sheath within which the sensing elementis housed, comprising the steps of:

a. providing a tubular sheath portion having spaced lead wires thereinand dielectric material intermediate the lead wires and sheath, an endportion of the lead wires protrudingfrom one end of the sheath,

b. providing a sensing element of resistance wire,

c. uniting the ends of the wire to respective ones of the lead wires,

d. locating a tubular sheath section of a length greater than the lengthof the element around the element and contiguous to that end of thesheath portion at which the lead wires protrude,

e. joining the contiguous ends of the sheath section and sheath portion,

f. filling the space between the element and sheath section withdielectric, refractory material,

g. securing closure means to the outer end of the sheath section,

h. swaging at least said sheath section to a smaller diameter followingsecurement of the closure means,

i. removing said closure means,

j. removing some of the refractory material to expose a part of theelement,

k. severing the element and trimming at least one of the cut endsthereof to provide a selected ohmic resistance for the element,

1. rejoining the ends of the element,

m. refilling the sheath section with refractory material to cover theelement, and

n. reclosing the outer end of the sheath section.

1. The method of manufacturing a resistance-temperature device whichincludes a metallic sheath within which the sensing element is housed,comprising the steps of: a. providing a tubular sheath portion havingspaced lead wires therein and dielectric material intermediate the leadwires and sheath, an end portion of the lead wires protruding from oneend of the sheath, b. providing a sensing element of resistance wire, c.uniting the ends of the wire to respective ones of the lead wires, d.locating a tubular sheath section of a length greater than the length ofthe element around the element and contiguous to that end of the sheathportion at which the lead wires protrude, e. joining the contiguous endsof the sheath section and sheath portion, f. filling the space betweenthe element and sheath section with dielectric, refractory material, g.securing closure means to the outer end of the sheath section, h.swaging at least said sheath section to a smaller diameter followingsecurement of the closure means, i. removing said closure means, j.removing some of the refractory material to expose a part of theelement, k. severing the element and trimming at least one of the cutends thereof to provide a selected ohmic resistance for the element, 1.rejoining the ends of the element, m. refilling the sheath section withrefractory material to cover the element, and n. reclosing the outer endof the sheath section.